There are major accumulations of oil sands at a number of global locations, including Canada, United States and Venezuela. Oil sands are basically a mixture of bitumen, mineral and water of variable bitumen content. Only those surficial deposits of the Athabasca region of Alberta, Canada are being exploited on a commercial scale at this time. In the deposits, the bitumen content varies up to about 18 wt.% and averages about 12 wt.%, water is usually about 3 to about 6 wt.% and the mineral content, predominantly quartz, ranges from about 84 to about 86 wt.%.
At the present time there is one commercial procedure for the recovery of bitumen from these deposits, known as the "hot water" process, although other procedures have been suggested. The recovery procedures attempt to separate the bitumen from the oil sand. The recovered bitumen is subjected to upgrading operations to form a synthetic crude oil.
The initial step of such upgrading operation is to subject the bitumen to a coking step. The coking step involves evaporating off volatiles from the bitumen to leave a solid carbonaceous material, known as "coke". The quantity of coke formed may vary, and is usually in the range of about 10 to 25 wt.% of the bitumen.
The coke so formed has a high sulphur content, usually about 5.5 to 6 wt.%, which can be oxidized to sulphur dioxide. The sulphur content of the coke inhibits its use as a source of thermal energy, unless elaborate and expensive sulphur dioxide removal equipment is used in association with the combustion gas stream.
Other heavy crude oils, which do not have substantial proportions of minerals associated therewith, and residua are also subjected to a initial coking step in upgrading the same, to result in sulphur-containing coke. Such heavy crude oils and residua are characterized by an API (American Petroleum Institute) gravity value of less than 25.degree., usually less than 20.degree., and occur in substantial deposits in Western Canada, California and the Orinoco basin in Venezuela.